"Climate change is one of the great global challenges of our time," the joint statement by the leaders of the 16 major economies said, adding that they all "recognised that deep cuts in global emissions will be necessary".

It said that all sides had agreed to "work constructively together to promote the success of the Copenhagen climate change conference", referring to the United Nations meeting scheduled for December 2009 aimed at reaching a new global treaty on reducing emissions.

Poor priority

India and China, however, refused to set specific targets for cutting emissions, saying they must put development priorities first.

Both countries argue that while their total emissions are growing fast on a per capita basis, they still lag well behind major polluters such as the US.

During Wednesday's major economies meeting, Manmohan Singh, the Indian prime minister, said that India's top priority was to help its poor and could not even consider quantitative restrictions on emissions.

"The imperative for our accelerated growth is even more urgent when we consider the disproportionate impact of climate change on us as a developing country," Singh said, according to a transcript of his speech.

He said India had "little choice but to devote even more and huge resources to adaption in critical areas of food security, public health and management of scarce water resources", he said.

Declaration

For his part Bush, who early in his presidency disputed many of the notions of climate change, said he backed the G8's declaration that greenhouse emissions should be slashed by 50 per cent by the middle of the century.

Protesters have condemned the summit as a holiday for G8 leaders [EPA]

But he stuck to the long-held US line that in order to be successful, plans to cut emissions must also include commitments from rapidly emerging economies like India and China.

"We made clear, and the other nations agreed, that they must also participate in an ambitious goal… to enable the world to successfully address climate change," the US president told reporters.

The G8 agreement to cut gas emissions in half by the middle of the century was welcomed as a major breakthrough by summit host Japan, but was swiftly condemned as meaningless by critics in the scientific community.

Alden Meyer, a spokesman for the Union of Concerned Scientists, said that without an agreement on a base year there was no way to measure if states were living up to commitments.

"They could have made progress here by being more specific on the near-term commitments that industrialised countries were willing to make to reduce their own emissions, but they don't have agreement on that," he said.